1. Field of the Invention
The present invention relates to a shift control method and a shift control system of an automatic transmission. More particularly, the present invention relates to a shift control method and a shift control system of an automatic transmission that control a shift from an N shift-speed achieved by engagements of first and second frictional elements to an N−3 shift-speed achieved by engagements of third and fourth frictional elements.
2. Description of Related Art
Generally, one frictional element is released and another frictional element is engaged according to a clutch-to-clutch shift control. However, during a specific skip-shifting process, two frictional elements may be released and two other frictional elements may be engaged.
Particularly, in the case of a kick-down 6 to 3 skip shift where three speed steps are shifted, generally two frictional elements must be released and two other frictional elements must be engaged. However, it is understood that shift control of releasing two frictional elements and engaging two other frictional elements is hard to be realized since the hydraulic pressure supplied to four frictional elements must be simultaneously controlled.
Therefore, much research on two shifting processes that are successively performed in the case of a skip shift, such as a 6 to 3 shift, has been conducted. For example, a 4 to 3 shift is performed after a 6 to 4 shift is completed so as to perform a 6 to 3 shift.
However, according to such a conventional 6 to 3 skip shift control method, since two shifting processes are successively performed, shifting time may be long. That is, since the 4 to 3 shift is performed after the 6 to 4 shift is completed, the shifting time may be long.
In addition, since the 4 to 3 shift is performed after the 6 to 4 shift is completed, the shifting process may not be smoothly performed and shift feel deteriorates.
To solve such problems, many shift control methods of an automatic transmission where a first shift from a 6 shift-speed to a 4 shift-speed and a second shift from the 4 shift-speed to a 3 shift-speed are overlapped have been researched. In such a shift control method, a second shift from an intermediate shift-speed between the 6 shift-speed and the 3 shift-speed to the 3 shift-speed is overlapped with a first shift from the 6 shift-speed to the intermediate shift-speed when a 6 to 3 shift signal is detected.
However, according to such a shift control method, an output torque of the intermediate shift-speed may be generated and a double shift feel may be felt since shifting from the 6 shift-speed to the 3 shift-speed is performed by way of the intermediate shift-speed. Further, shift shock may occur.
More concretely, in a case in which a shift is performed according to the conventional shift control method, a turbine speed remains at the intermediate shift-speed for a while, and output torque fluctuates seriously, as shown in FIG. 7.
The information disclosed in this Background of the Invention section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art.